1. Field of the Invention
The present invention relates generally to the field of explosive shaped charges. More specifically, the present invention relates to a composition of material for use as a liner in a shaped charge, particularly a shaped charge used for oil well perforating.
2. Description of the Related Art
Shaped charges are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined ones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Shaped charges known in the art for perforating wellbores can include a housing, a quantity of high explosive of a composition such as HMX, RDX or HNS inserted into the housing, and a liner which is inserted onto the high explosive. The liner is typically formed into a generally conical shape by compressing powdered metal. The powdered metal typically used is primarily composed of copper. The powdered metal can include a fractional amount of lead mixed therewith, usually not more than twenty percent by weight. Alternatively, as disclosed in U.S. Pat. No. 5,221,808 issued to Werner et al for example, the lead can be substituted by bismuth.
When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a "jet". The jet penetrates the casing, the cement and a quantity of the formation. The quantity of the formation which may be penetrated by the jet can be estimated for a particular design shaped charge by test detonation of a similar shaped charge under standardized conditions which are specified in "Recommended Practice No. 43" ("RP-43") published by the American Petroleum Institute. The test procedure specified in RP-43 includes using a long cement "target" through which the jet partially penetrates. The depth of jet penetration through the RP-43 specification target for any particular type of shaped charge has a high degree of correspondence to the depth of jet penetration of a similar type charge through an earth formation.
In order to provide perforations which have efficient hydraulic communication with the formation, it is known in the art to design shaped charges in various ways to provide a jet which can penetrate a large quantity of formation, the quantity usually referred to as the "penetration depth" of the perforation. One method known in the art for increasing the penetration depth is to increase the quantity of explosive provided within the housing. A drawback to increasing the quantity of explosive is that some of the energy of the detonation is expended in directions other than the direction in which jet is expelled from the housing. As the quantity of explosive is increased, therefore, it is possible to increase the amount of detonation-caused damage to the wellbore and to equipment used to transport the shaped charge to the depth within the wellbore at which the perforation is to be made.
It is also known in the art to design the shape of the liner in various ways so as to maximize the penetration depth of the shaped charge for any particular quantity of explosive. Even if the shape of the liner were optimized, the amount of energy which can be transferred to the liner for making the perforation is necessarily limited by the quantity of explosive.
The copper/bismuth liner disclosed in the Werner et al '808 patent can reduce the environmental risk believed to be associated with lead deposited within the perforation by lead-containing charge liners, but as stated in the '808 patent, column 2 lines 48-49, the combination of bismuth and copper in the liner provides a shaped charge which "can shoot as well as the standard shaped charge" the standard charge being one which includes copper and lead in the liner material. Bismuth substituted for lead in the liner material does not provide increased penetration depth.
It is also known in the art to alter the composition of the liner to include powdered tungsten in substitution of some of the powdered copper in order to improve the performance of the shaped charge. Tungsten has been substituted in liners to compositions having as much as 35 percent by weight of tungsten. Those skilled in the art believed that substitution of higher fractional weights of tungsten in the liner material would not increase performance of the shaped charge because tests performed using liner tungsten concentrations exceeding 35 percent typically showed that the performance of the charges decreased. Therefore, liner compositions exceeding 35 percent by weight of tungsten were not used.
It is an object of the present invention to provide a liner material for a shaped charge which increases the penetration depth of the shaped charge by substitution of tungsten for most or all of the copper in the liner material.